Holocene Rice Rats (Genus Oryzomys) from the Upper Mississippi River Drainage Basin

Abstract

The expansion and collapse of the geographic range of the Texas rice rat (Oryzomys texensis) in the upper Mississippi River drainage basin at the end of the Holocene was a unique event in North American mammals. In a period of about 4000 years with a point of origin near the American Bottom in Illinois, these small rodents extended their geographic range in a straight-line distance of over 950 km to the west into Nebraska and the same distance to the east into Pennsylvania. Then in less than 400 years this range expansion collapsed back to a point where the northern-most edge of the modern geographic range of these rice rats is in southern Illinois. It is concluded that no single factor lead to this geographic range expansion, but it was a complex interplay of changes in Native American populations, culture, foodways, riverine habitats, and climate along with the impact of kleptoparasitism and passive anthropochory. The collapse of the expanded geographic range of Texas rice rats appears to have occurred between AD 1400 and AD 1600, but it did not occur simultaneously throughout the geographic range. This was not an orderly range contraction, but a collapse of populations in place with many local extinction events. These rice rat populations declined beginning with the onset of the Little Ice Age, which brought a colder and wetter climate that caused crop failures resulting from droughts, cold temperatures, or shortened growing seasons. These conditions stressed the dietary reserves of the human populations and thereby the rice rat populations. These conditions, particularly droughts, were harmful to the growing of maize, which served as the primary food resource of the Native Americans and the associated populations of rice rats. It is proposed that the pre-1910 records of rice rat from unusual localities compared to the modern geographic range in southwestern Ohio, Kentucky, and Kansas represent the final extinction events of these Holocene rice rat populations.